An internal combustion engine has a number of cylinders, each connected to an intake manifold by at least one intake valve, and to an exhaust manifold by at least one exhaust valve. The intake manifold is supplied with fresh air (i.e. from outside) by a feed conduit fitted with a throttle valve, and is connected to the cylinders by respective intake conduits, each terminating at least one intake valve.
A choke device has recently been proposed to adjust the cross section of the intake conduits as a function of engine speed (i.e. drive shaft rotation speed). At low speed, the airflow section of the intake conduits is reduced to generate turbulence in the air intake and so improve mixing of the air and fuel in the cylinders, so that all the injected fuel is burnt, thus reducing combustion-generated pollutant emissions. At high speed, the airflow section of the intake conduits is maximized to fill the cylinders completely and achieve maximum power.
Choke devices as described above may be tumble types, referred to simply as tumble devices, or swirl types, referred to simply as swirl devices. A tumble device employs, for each intake conduit, a choke body movable to and from an active position in which the choke body reduces the cross section of the intake conduit. In a swirl system, each intake conduit comprises two parallel, side by side channels, and, for each intake conduit, a choke body is used which is movable to and from an active position completely closing one of the two intake conduit channels.
In currently marketed choke devices, all the choke bodies are fitted to a common shaft to rotate together to and from the work position under the control of a common actuator, which simultaneously and synchronously controls the position of all the choke bodies. The actuator is defined by an electric motor having a rotor connected mechanically to the common shaft. This solution, however, is relatively heavy, is relatively bulky (also on account of the electronics controlling the electric motor), and is only sufficiently, though not particularly, dependable (due to frequent malfunctioning of the electronics controlling the electric motor). In currently marketed engines, it has also been proposed to operate the common shaft using a pneumatic actuator. This solution, however, is also relatively heavy and, above all, extremely bulky.
EP1568866 discloses a choke valve for an intake manifold with a retracting tumble system for an engine provided with a number of cylinders. The choke valve is adapted to vary the passage section of the air through an intake duct which connects each cylinder with the intake manifold and is regulated by at least one intake valve; the choke valve comprises a butterfly mounted to rotate about an axis of rotation disposed transversely and externally with respect to the corresponding intake duct under the thrust of a corresponding rotary electric motor which comprises a stator and a rotor housed directly in the butterfly.